Various communications systems utilize quadrature amplitude modulation (QAM) for transmission of relatively high data rate information within a limited transmission bandwidth. Typically, QAM communications systems use a fixed symbol constellation for all transmissions, e.g., 16-ary QAM or sixteen positions or symbol points within a constellation. It is desirable that receivers in such communications systems have some type of automatic gain control (AGC) circuitry so that the ratio of received QAM signal to background noise is optimized and the received QAM signal suffers minimal distortion as it is processed by the receiver. This condition is achieved using AGC circuitry that maintains a constant signal level out of the tuner portion of the QAM receiver as the input RF signal fades or is attenuated. In a typical QAM receiver, the AGC circuitry contains two stages, an IF AGC within an IF stage of the receiver and an RF AGC within an RF stage of the receiver, where the IF AGC circuitry follows the RF AGC circuitry. As such, the IF AGC is maintained at a constant gain until the RF AGC applies a maximum gain to the input RF signal. Once the maximum RF AGC gain is attained, the IF AGC circuitry becomes operational to further amplify the IF signal.
However, when a signal is amplified at an amplifier's gain limit, the amplifier produces intermodulation distortion. In QAM, this distortion typically effects the outermost constellation points. Given an input signal with a low signal-to-noise ratio, a QAM receiver has difficulty demodulating the outermost constellation points when the IF and RF AGC circuit gain is near or at its maximum limit.
Thus, there is a need for a system that provides optimal gain control in both the RF and IF stages of a QAM receiver to minimize intermodulation distortion.